1. Field of the Invention
The present invention relates to a method for forming a substrate for an optical recording medium on which optical recording and reproducing or playback are carried out by a laser beam or the like.
2. Related Background Art
Heretofore, an optical recording medium has been formed by providing a recording layer on a substrate transparent to light such as a semiconductor layer by which recording and playback of information are carried out, and then laminating a protective layer on the recording layer. On the surface of this substrate for the optical recording medium, there are engraved fine preformats of micron order or submicron order such as grooves for tracking and address information pits. As conventional methods for forming these preformats, for example, an injection molding method and a compression molding method are known. However, these methods are unsatisfactory in points of mass productivity and cost.
In order to solve this problem, the present applicant has suggested, in EP-A-0 369 780 and EP-A-0 369 781, methods for preparing substrate sheets for optical recording media which comprises the steps of melting and extruding a resin to mold resin sheets, and then pressing the melted resin sheets 104 between a molding roll 102, having a preformat pattern on the surface thereof, and an oppositely disposed mirror roll 101 to transfer preformats thereto as shown in FIG. 3.
According to the above-mentioned methods, the substrate sheets for optical recording media can be continuously formed which are excellent in transferability of the preformat and which controls double refraction which will be the cause of the decline in a C/N value, even when a resin such as polycarbonate which easily causes the double refraction is used.
However, as a result of the investigations on the above-mentioned methods, it has been found that the double refraction and the transfer accuracy of the substrate sheet for optical recording media change owing to the increase or decrease of the amount of the extruded melted resin attributable to the slight alteration of conditions at the time of the molding, for example, the heating temperature of an extruder or a T-die. This increase or decrease of the amount of the extruded melted resin causes the change of the thickness of the melted resin sheet pressed between the molding roll 102 and the mirror roll 101. On the other hand, the molding roll 102 and the mirror roll 101 which can be used are each made usually from a steel roll plated with chromium, and therefore, it can be considered that when the thick melted resin sheet 104 is extruded from a T-die 105 and then pressed by the rolls, excessive pressure is applied to the resin sheet, with the result that the value of the double refraction and the transfer accuracy of the substrate sheet for optical recording media are uneven.
In order to solve the above-mentioned problem, there is a technique for covering the surface of the mirror roll 101 with a resin. According to this technique, when the polycarbonate sheet coming from the T-die is molded by a resin roll and the molding roll, the resin is not forcedly pressed, so that the resultant substrate sheet for optical recording media is less strained in the process of the sheet's preparation.
However, in the case that the resin roll is used, it is necessary to subject the surface of the resin roll to a mirror processing, but the resin roll is poorer in durability as compared with a metal roll. Thus, as molding is repeatedly carried out, the surface of the resin is damaged, and the surface of the roll must often be polished or the roll is required to be exchanged.